Oxygen molecule has a negative effect on perovskite solar cells, which has been investigated experimentally. However, detailed theoretical research is still rare. This study presents a microscopic view to reveal the interaction mechanism between O-2 and perovskite based on the first-principles calculation. The results show that O-2 is adsorbed on the (100) surface of MAPbI(3) perovskite mainly by Van der Waals force. O-2 adsorption makes the MAPbI3 surface generate a small number of positive charges, which leads to the increase of the work function of the MAPbI(3) surface. This is in agreement with the experimental measurement. And increased work function of MAPbI(3) surface is not beneficial to electron transfer from perovskite to electronic extraction layer (such as TiO2). Comparison of the density of states (DOS) of the clean (100) surface and the adsorbed system shows that an in-gap state belonging to O-2 appears, which can explain the phenomenon observed from experiments that electron transfers from the surface of perovskite to O-2 to form superoxide. The theoretical power conversion efficiency of the system with and without O-2 adsorption is evaluated, and it turns out that the power conversion efficiency of the system with O-2 adsorption is slightly lower than that of the system without O-2 adsorption. This result indicates that avoiding the introduction of O2 molecules between perovskite and electronic extraction layer is beneficial to the perovskite solar cell. (C) 2017 Elsevier B.V. All rights reserved.